A typical field emission display 8 is shown in FIG. 1. The display 8 includes a substrate or base plate 10 having a conductive layer 12 formed thereon. A plurality of emitters 14 are formed on the layer 12. Also formed on the layer 12 is an electrically insulating layer 16 having a conductive layer formed thereon. The conductive layer formed on the insulating layer 16 typically functions as an extraction grid 18 to control the emission of electrons from the emitters 14, and is typically formed from metal. An anode 20, which acts as a display screen and has a cathodoluminescent coating 22 formed on an inner surface thereof, is positioned a predetermined distance from the emitters 14. Typically, a vacuum exists between the emitters 14 and the anode 20. A power source 24 generates a voltage differential between the anode 20 and the substrate 10, which acts as a cathode. Also, a voltage applied to the extraction grid 18 generates an electric field between the grid and the substrate 10. An electrical path is provided to the emitters 14 via the conductive layer 12 such that in response to this electric field, the emitters 14 emit electrons. The emitted electrons strike the cathodoluminescent coating 22, which emit light to form a video image on the display screen. Examples of such field emission displays are disclosed in the following U.S. patents, all of which are incorporated by reference:
U.S. Pat. No. Issue Date 3,671,798 June 20, 1972 3,970,887 July 20, 1976 4,940,916 July 10, 1990 5,151,061 September 29, 1992 5,162,704 November 10, 1992 5,212,426 May 18, 1993 5,283,500 February 1, 1994 5,359,256 October 25, 1994
Field emission displays, such as the field emission display 8 of FIG. 1, often suffer from technical difficulties relating to the control of the current flowing through the emitters 14. For example, due to the relatively small dimensions of the components involved, manufacturing defects are common in which an emitter 14 is shorted to the extraction grid 18. Because the voltage difference between the substrate 10 and the anode 20 is typically on the order of 1000 volts or more and a high electric field exists between tip 14 and substrate 10, the above defect can cause a current to flow through the emitter 14 that is sufficient to destroy not only the shorted emitter 14 itself, but other surrounding emitters 14 and circuitry as well. Thus, such a current draw will typically result in damage to, if not complete destruction of, the field emission display. Furthermore, if the current through the emitters 14 is unregulated, it is virtually impossible to control the emission level of the emitters 14, and thus the brightness level of the field emission display 8.
Efforts to solve the above limitations have focused on providing a resistance between the conductive layer 12 and the emitters 14 to limit the current flow through the emitters 14. An example of such a resistance is disclosed in U.S. Pat. No. 4,940,916, was previously incorporated by reference. One limitation to this scheme, however, is that the resistivity (which is the inverse of the conductivity) of the resistive layer often fluctuates in response to conditions that vary during the operation of the field emission display, particularly the varying light intensity resulting from the emitted electrons striking the cathodoluminescent coating 22 or from ambient light.